With the load on demand feature, data in tree is not loaded until the requested data is needed in view.

<zk>			                                 
	<zscript><![CDATA[
		TreeModel btm = new demo.tree.load_on_demand.BinaryTreeModel(
				new ArrayList(
						new demo.data.BigList(1000)));
	]]></zscript>
	<tree id="tree" model="${btm}"/>
</zk>
			

package demo.tree.load_on_demand;

import java.util.ArrayList;

import org.zkoss.zul.AbstractTreeModel;
import org.zkoss.zul.ext.TreeSelectableModel;

public class BinaryTreeModel<T> extends AbstractTreeModel<T> implements TreeSelectableModel {

	private static final long serialVersionUID = 7822729366554623684L;
	
	private ArrayList<T> _tree = null;

	/**
	 * Constructor
	 * 
	 * @param tree
	 *            the list is contained all data of nodes.
	 */
	public BinaryTreeModel(ArrayList<T> tree) {
		super(tree.get(0));
		_tree = tree;
	}

	// TreeModel //
	public T getChild(Object parent, int index) {
		int i = _tree.indexOf(parent) * 2 + 1 + index;
		if (i >= _tree.size())
			return null;
		else
			return _tree.get(i);
	}

	public int getChildCount(Object parent) {
		int count = 0;
		if (getChild(parent, 0) != null)
			count++;
		if (getChild(parent, 1) != null)
			count++;
		return count;
	}

	public boolean isLeaf(Object node) {
		return (getChildCount(node) == 0);
	}

	/**
	 * @since 5.0.6
	 * @see org.zkoss.zul.TreeModel#getIndexOfChild(java.lang.Object,
	 *      java.lang.Object)
	 */
	@Override
	public int getIndexOfChild(Object arg0, Object arg1) {
		return 0;
	}

}

package demo.data;

import java.util.AbstractList;

public class BigList extends AbstractList<Integer> {

	private int size;

	public BigList(int sz) {
		if (sz < 0)
			throw new IllegalArgumentException("Negative not allowed: " + sz);
		size = sz;
	}

	public int size() {
		return size;
	}

	public Integer get(int j) {
		return Integer.valueOf(j);
	}

}